Caissons are structures embedded into the earth that may serve as reservoirs, mine shafts, pits, piles, or the like. Sinking caissons are also their own best excavation tool because their massive weight is used to embed or sink them into the earth. A non-limiting example of a typical sinking caisson is set forth below.
A typical sinking caisson is generally hollow, may be open or closed, and may be circular or of any other suitable shape. The caisson is preferably a multiple stage hollow cylinder, or annulus, composed of steel bar reinforced concrete and usually about 30 to 150 feet in diameter and with a side wall about 5 to 10 feet thick. The caisson includes a first stage comprised of a concrete, partially wedge-shaped in cross section cutting shoe. The shoe is tapered from a shelf at an outside surface to a cutting tip which is only about 1 to 2 feet thick to pierce the earth.
The cutting shoe is cast in-situ on a circular launch pad typically of timbers lying on a circular bed of crushed rock, and one or more additional stages of concrete rings may be cast in-situ on the cutting shoe before sinking begins. Each stage of a concrete ring may weigh hundreds of thousands or even millions of pounds depending on the diameter and wall thickness of the caisson and the soil conditions into which it will sink. Sinking of the caisson is launched by removing the timbers so that the caisson begins to sink into the earth under its own weight. The caisson continues to sink, increasing its embedded length, until a sinking force of the caisson is equalized by earthen drag forces imposed on the interior and exterior surfaces of the caisson including the shoe.
An excavator is disposed within the periphery of the caisson on an earthen bench. The excavator removes earth from a trench adjacent the caisson interior surface to reduce the drag force acting on the interior of the caisson at the shoe. The excavator also removes earth from the bench to reduce the exterior drag force by allowing controlled floor heave from outside to inside earthen flow around and under the tip of the shoe. Lubricant materials may be pumped to the outside perimeter of the caisson to further reduce the exterior drag force.
Successive rings of concrete are poured or cast (with suitable forms) on top of one another to increase the caisson length so the caisson sinks into the earth. This also increases the caisson weight and sinking force, causing the caisson to sink further into the earth. The excavation and construction is continued so that the caisson tip reaches a desired final depth typically resting on bed rock.
But the sinking caisson may tilt as it sinks into the earth because of imperfections in excavation as well as the heterogeneous or varying structure of the earth. Current attempts to control such tilt include use of time-consuming traditional surveying manpower, equipment, and methods, including multi-person crews, transits, and referencing of surface earth benchmarks located outside of and distal from the caisson and the continuously moving top of the caisson. But such methods are prone to many errors inherent in the surveying equipment and in multi-person surveying techniques. Therefore, such methods are unsatisfactory because they are unable to rapidly and accurately assess caisson tilt. Thus, caisson sinking is inadequately controlled, thereby resulting in cracked or otherwise structurally compromised caissons.